Steel wire having a structure of a strain-hardened lower bainite type and method for producing such wire

ABSTRACT

A metal wire having the following features: 
     (a) it is formed, at least in part, by a steel having a carbon content of at least 0.1% and at most 0.6% and a boron content of less than 8 ppm; 
     (b) the steel of the wire has a strain-hardened lower bainite type structure (7); 
     (c) the diameter of the wire varies from 0.10 to 0.40 mm; 
     (d) the resistance to rupture of the wire is at least equal to 2800 MPa; 
     (e) the elongation upon rupture of the wire is at least equal to 0.4%. 
     The method according to the invention for producing this wire consists in strain hardening a machine wire having 28% to 90% proeutectoid ferrite and 72% to 10% perlite, thereupon carrying out a heat treatment to obtain a structure of lower bainite type, then effecting a strain hardening on the wire, the temperature of the wire upon the strain hardening being less than 0.3 T F , T F  being the melting point of the steel expressed in Kelvin.

BACKGROUND OF THE INVENTION

The present invention relates to metal wires and the processes ofobtaining them. These wires are used, for instance, to reinforcearticles of plastic or rubber, in particular tubes, belts, plies andautomobile tires.

The wires of this type which are presently/currently used are formed ofsteel containing at least 0.6% carbon, this steel having astrain-hardened perlitic structure. The rupture strength of these wiresis about 2800 MPa (megapascals); their diameter varies generally from0.15 to 0.35 mm, and their elongation upon rupture is between 0.4 and2%. These wires are made by drawing an initial wire, known as a "machinewire", the diameter of which is about 5 to 6 mm, the structure of thismachine wire being a hard structure formed of perlite and ferrite with ahigh content of perlite, generally more than 72%. Upon the production ofthis wire, the drawing is interrupted at least once in order to carryout one or more heat treatments which make it possible to regenerate theinitial structure.

This process has the following drawbacks:

the raw material is expensive, since the carbon content is relativelyhigh;

the parameters cannot be easily modified; in particular, the diameter ofthe machine wire and the final diameter are maintained within strictlimits, the process therefore lacking flexibility;

the great hardness of the machine wire due to its strongly perliticstructure makes drawing prior to the heat treatment difficult, so thatthe rate of deformation ε of this drawing is necessarily less than 3;furthermore, the speed of this drawing is low and there may be breaks ofthe wire upon this operation.

Furthermore, the wires themselves sometimes have insufficient resistanceto rupture and their resistance to fatigue is limited, due probably to adamaging of these wires upon the drawing before the heat treatment as aresult of the great hardness of the machine wire.

The Japanese patent application published under No. 54-79119 describes aprocess of preparing a boron steel wire of bainite structure by heatingin a fluidized bed. The wires obtained have poor mechanical properties.

The object of the present invention is to propose a strain-hardenedmetal wire of non-perlitic structure having a resistance to rupture andan elongation upon rupture at least as high as the known strain-hardenedperlitic steel wires, and less damage than the known wires.

Another object of the invention is to propose a process not having theaforementioned drawbacks for the production of this wire.

The metal wire in accordance with the invention has the followingcharacteristics:

(a) it is formed at least in part by a steel having a carbon content ofat least 0.1% and at most 0.6%, and a boron content of less than 8 ppm(parts per million);

(b) the steel of the wire has a strain-hardened lower bainite typestructure;

(c) the diameter of the wire varies from 0.10 to 0.40 mm;

(d) the resistance to rupture of the wire is at least 2800 MPa;

(e) the elongation upon rupture of the wire is at least 0.4%.

The process of the invention for the production of this wire ischaracterized by the following features:

(a) a steel machine wire having a carbon content of at least 0.1% and atmost 0.6% and a boron content of less than 8 ppm (parts per million) isstrain-hardened, said steel comprising 28% to 90% proeutectoid ferriteand 72% to 10% perlite; the deformation ratio ε of this strain hardeningbeing at least equal to 3;

(b) the strain hardening is stopped and a single structural heattreatment is carried out on the strain-hardened wire; this treatmentconsists in heating the wire to above the AC3 transformation point inorder to impart it a homogeneous austenite structure, then cooling itrapidly to a temperature of between 350° C. and 450° C., the rate ofthis cooling being at least equal to 250° C./second, and maintaining itwithin this temperature range for a period of time of at least 30seconds so as to obtain a structure of lower bainite type having carbideprecipitates distributed practically uniformly in a ferrite matrix;

(c) the wire is cooled to a temperature below 0.3 T_(F), T_(F) being themelting point of the steel expressed in Kelvin;

(d) a strain hardening is carried out on the wire which has undergonethis heat treatment, the temperature of the wire upon the strainhardening being less than 0.3 T_(F), the deformation ratio e of thisstrain hardening being at least equal to 3.

The invention also concerns assemblies comprising at least one wire inaccordance with the invention.

The invention also concerns articles reinforced at least in part bywires or assemblies in accordance with the above definitions, sucharticles consisting, for instance, of tubes, belts, plies and automobiletires.

The invention will be easily understood on basis of the followingembodiments and the diagrammatic figures relating to these examples.

DESCRIPTION OF THE FIGURES IN THE DRAWING

In the drawing:

FIG. 1 shows the structure of the steel of a wire before heat treatment,upon the carrying out of the process of the invention;

FIG. 2 shows the structure of the steel of a wire after heat treatmentupon the carrying out of the process of the invention;

FIG. 3 shows the structure of the steel of a wire in accordance with theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following specification, all percentages and parts per million ofcomposition indicated are parts by weight and the rupture resistancemeasurements and measurements of elongation at rupture are carried outin accordance with AFNOR method NFA 03-151.

By definition, the deformation ratio ε of a strain hardening is given bythe formula ε=Ln (So/Sf) being the neper logarithm, So being the initialcross section of the wire before the strain hardening, and Sf being thecross section of the wire after this strain hardening.

The object of the following examples is to describe the preparation andproperties of three wires in accordance with the invention.

In these examples, a machine wire of a diameter of 5.5 mm which has notbeen strain-hardened is used. This machine wire is formed of a steel theproperties of which are as follows:

    ______________________________________                                        carbon content:    0.4%                                                       boron content:     less than 8 ppm;                                           manganese content: 0.4%;                                                      silicon content:   0.2%;                                                      phosphorus content:                                                                              0.015%;                                                    sulfur content:    0.02%;                                                     aluminum content:  0.015%;                                                    nitrogen content:  0.005%;                                                    chromium content:  0.05%;                                                     nickel content:    0.10%;                                                     copper content:    0.10%;                                                     molybdenum content:                                                                              0.01%;                                                     proeutectoid ferrite content:                                                                    53%;                                                       perlite content:   47%;                                                       melting point of the steel, T.sub.F :                                                            1795        K                                              resistance to rupture R.sub.m :                                                                  700         MPa;                                           elongation at rupture A.sub.r :                                                                  17%                                                        ______________________________________                                    

Three wires in accordance with the invention are made with this machinewire in the following manner:

Example 1

The machine wire is descaled, coated with a drawing soap, for instanceborax, and drawn dry so as to obtain a wire of a diameter of 1.1 mm,which corresponds to a deformation ratio ε of slightly more than 3.2.

The drawing is easily effected due to the relatively ductile structureof the machine wire. By way of example, a non-strain-hardened steel of0.7% carbon has a resistance to rupture R_(m) of about 900 MPa and anelongation at rupture of about 8%; in other words, it is definitely lessductile.

The drawing described above is effected at a temperature of less than0.3 T_(F) ; for purposes of simplification, although this is notindispensable, the drawing temperature may possibly be equal to orexceed 0.3 T_(F).

FIG. 1 is a longitudinal section through a portion 1 of the structure ofthe wire thus obtained. This structure is formed of elongated blocks 2of cementite and elongated blocks 3 of ferrite, the largest dimension ofthese blocks being oriented in the direction of drawing.

The following heat treatment is then carried out on the wire thusobtained:

the wire is heated to bring it to 900° C., that is to say above the AC3transformation point, and it is held at this temperature for 1 minute soas to obtain a homogeneous austenite structure;

the wire is then cooled to 400° C. in a salt bath in less than 2seconds, and then maintained at this temperature for 1 minute, whereuponit is cooled to about 20° C., that is to say to room temperature.

FIG. 2 is a section through a portion 4 of the structure of the wirethus obtained. This structure, of lower bainite type, is formed ofcarbide precipitates 5 distributed practically uniformly in a ferritematrix 6. This structure is obtained by the preceding heat treatment andit is retained upon cooling to room temperature. The precipitates 5generally are of sizes at least equal to 0.005 μm (micrometer) and atmost equal to 0.5 μm.

The wire thus obtained by this thermal treatment and this cooling toroom temperature is covered with a layer of brass. The thickness of thislayer of brass is slight (on the order of μm) and is negligible ascompared with the diameter of the wire before the brass coating. Wetdrawing of this wire is then effected so as to obtain a final diameterof 0.2 mm, which corresponds practically to ε=3.4. The wire drawing isfacilitated by the layer of brass. The temperature of the wire upon thisdrawing is necessarily less than 0.3 T_(F).

FIG. 3 is a longitudinal section through the portion 7 of this wireaccording to the invention which is thus obtained. This portion 7 has astructure of strain-hardened lower bainite type formed of carbides 8 ofelongated shape which are practically parallel to each other and thelargest dimension of which is oriented along the axis of the wire, thatis to say along the direction of drawing indicated by the arrow F inFIG. 3. These carbides 8 are arranged in a strain-hardened ferritematrix 9.

This wire in accordance with the invention has a resistance to ruptureof 3200 MPa and an elongation upon rupture of 0.7%.

Example 2

The machine wire is descaled, coated with a layer of drawing soap, forinstance borax, and drawn dry to obtain a wire of a diameter of 0.9 mmwhich corresponds to a deformation ratio ε slightly greater than 3.6.The structure obtained is analogous to that shown in FIG. 1. Thefollowing heat treatment is then carried out on the wire thus obtained:

the wire is heated in the same manner as in Example 1 so as to obtain ahomogeneous austenite structure;

the wire is then cooled to 370° C. in less than 2 seconds and held atthis temperature for 90 seconds, whereupon it is cooled to roomtemperature.

The structure obtained is similar to that shown in FIG. 2. The wire isthen coated with brass and drawn in a manner similar to Example 1 so asto obtain a final diameter of 0.17 mm, which corresponds practically toε=3.3. The temperature of the wire upon this drawing is less than 0.3T_(F). The wire in accordance with the invention thus obtained has astructure similar to that shown in FIG. 3.

This wire has a resistance to rupture equal to 3000 MPa and anelongation upon rupture of 0.9%.

Example 3

A wire in accordance with the invention is produced in the same manneras in Example 1, but with the difference that the drawing carried outafter the heat treatment is continued to a final diameter of 0.17 mm,which corresponds practically to ε=3.7. This wire in accordance with theinvention has a resistance to rupture equal to MPa and an elongationupon rupture equal to 0.7%. The intermediate structures and the finalstructure are similar to the structures previously described.

The invention has the following advantages:

one starts from a machine wire of low carbon content and therefore oflow cost;

there is great flexibility with respect to the selection of thediameters of the wires; thus, for instance, one can use machine wiresthe diameter of which is substantially greater than 6 mm, which furtherreduces the cost, and wires of very different diameter can be produced;

the drawing before the structural heat treatment is relatively easy sothat the deformation ratio e of this drawing can be greater than 3;furthermore, this drawing can be effected at high speeds; finally, thefrequency of wire breaks and of changes of dies is reduced, whichfurther decreases the cost;

the wire obtained has a resistance to rupture and an elongation uponrupture of values at least equal to those of conventional wires, whichtherefore results in an energy of rupture at least equal to that of theconventional wires; PG,12

the wire is less damaged upon the drawing before heat treatment;

the wire obtained has better resistance to corrosion than theconventional wires, as a result of its low carbon content.

The steel of the wire according to the invention preferably has a carboncontent of at least 0.2% and at most 0.5%.

The steel of the wire according to the invention, and therefore theinitial machine wire, preferably have the following composition: 0.3%≦Mn≦0.6%; 0.1%≦Si≦0.3%; P≦0.02%; S≦0.02%; Al≦0.02%; N≦0.006%.

In the steel of the wire according to the invention and therefore in theinitial machine wire, one advantageously has the following ratios:Cr≦0.06%; Ni≦0.15%; Cu≦0.15%; Mo≦0.015%.

In the process of the invention, one preferably has at least one of thefollowing characteristics:

the initial machine wire has a carbon content of at least 0.2% and atmost 0.5%;

the initial machine wire has a proeutectoid ferrite content of at least41% and at most 78% and a perlite content of at least 22% and at most59%;

the deformation ratio e upon the strain hardening before the structuralheat treatment is at most equal to 6;

the deformation ratio e upon the strain hardening after the structuralheat treatment is at most equal to 4.5.

In the examples described above, the wire was coated with brass afterthe heat treatment in order to facilitate its drawing, but the inventioncovers cases in which drawing materials other than brass are used, forinstance copper, zinc and ternary, copper-zinc-nickel,copper-zinc-cobalt and copper-zinc-tin alloys, these materials beingother than steel.

The strain hardening of the wire in the preceding examples is effectedby drawing, but other techniques are possible, for instance rolling,possibly combined with a drawing, in the case of at least one of thestrain hardening operations.

Of course, the invention is not limited to the embodiments describedabove.

We claim:
 1. A metal wire characterized by the following features:(a) itis formed, at least in part, of a steel having a carbon content of atleast 0.1% and at most 0.6% and a boron content of less than 8 ppm(parts per million); (b) the steel of the wire has a structurestrain-hardened lower bainite; (c) the diameter of the wire varies from0.10 to 0.40 mm; (d) the resistance to rupture of the wire is at least2800 MPa; (e) the elongation at rupture of the wire is at least 0.4% 2.A metal wire according to claim 1, characterized by the fact that thesteel has a carbon content of at least 0.2% and at most 0.5%.
 3. A metalwire according to claim 1, characterized by the fact that the steelsatisfies the following relationships: 0.3%≦Mn≦0.6%; 0.1% ≦Si≦0.3%;P≦0.02%; S≦0.02%; Al≦0.02%; N≦0.006%.
 4. A metal wire according to claim3, characterized by the fact that the steel satisfies the followingrelationships: Cr≦0.06%; Ni≦0.15%; Cu≦0.15%; Mo≦0.015%.
 5. A metal wireaccording to claim 1, characterized by the fact that it is coated with ametal layer other than steel.
 6. A metal wire according to claim 5,characterized by the fact that it is coated with a layer of brass.
 7. Anassembly comprising at least one wire according to claim
 1. 8. Anarticle reinforced with at least one wire according to claim
 1. 9. Anarticle reinforced with at least one assembly according to claim
 7. 10.An article according to claim 9, characterized by the fact that it is anautomobile tire.
 11. A method of producing a metal wire comprising thesteps of:(a) strain hardening a steel machine wire having a carboncontent of at least 0.1% and at most 0.6% and a boron content of lessthan 8 ppm (parts per million), said steel comprising 28% to 90%proeutectoid ferrite and 72% to 10% perlite and the deformation ratio εof the strain hardening being at least equal to 3; (b) stopping thestrain hardening and then carrying out a single structural heattreatment on the strain hardened wire; said heat treatment consisting ofheating the wire to above the AC3 transformation point in order toimpart to it a homogeneous austenite structure, then cooling it rapidlyto a temperature of between 350° C. and 450° C. at a rate of at least250° C./second, and maintaining the wire within this temperature rangefor a period of time of at least 30 seconds so as to obtain a materialof lower bainite structure having carbide precipitates distributedpractically uniformly in a ferrite matrix; (c) cooling the wire to atemperature below 0.3 T_(F), T_(F) being the melting point of the steelexpressed in Kelvin; and (d) carrying out a second strain hardening onthe cooled wire while maintaining the temperature of the wire during thestrain hardening at less than 0.3 T_(F), the deformation ratio ε of thesecond strain hardening being at least equal to
 3. 12. A methodaccording to claim 11, characterized by the fact that the machine wirehas a carbon content of at least 0.2% and at most 5%.
 13. A methodaccording to claim 11, characterized by the fact that the machine wiresatisfies the following relationships: 0.3%≦Mn≦0.6%; 0.1%≦Si≦0.3%;P≦0.02%; S≦0.02%; Al≦0.02%; N≦0.006%.
 14. A method according to claim13, characterized by the fact that the machine wire satisfies thefollowing relationships: Cr≦0.06%; Ni≦0.15%; Cu≦0.15%; Mo≦0.015%.
 15. Amethod according to claim 11, characterized by the fact that a metalcoating other than steel is effected on the wire after the structuralheat treatment before strain hardening.
 16. A method according to claim15, characterized by the fact that said coating is a coating of brass.17. A method according to claim 11, characterized by the fact that themachine wire has a proeutectoid ferrite content of at least 41% and atmost 78% and a perlite content of at least 22% and at most 59%.
 18. Amethod according to claim 11, characterized by the fact that thedeformation ratio ε upon the strain hardening before the structural heattreatment is at least 3 and at most equal to
 6. 19. A method accordingto claim 11, characterized by the fact that the deformation ratio ε uponthe strain hardening after the structural heat treatment is at least 3and at most 4.5.
 20. A method according to claim 11, characterized bythe fact that at least one strain hardening is effected, at least inpart by drawing.
 21. A method according to claim 11, characterized bythe fact that the structure of lower bainite type obtained after therapid cooling is such that the carbide precipitates have, in general,dimensions of at least 0.005 μm (micrometer) and at most 0.5 μm.